Directed Assembly of Au Nanoparticles onto Planar Surfaces via

Aug 2, 2005 - Biophysics, Johannes Kepler University of Linz, Altenbergerstrasse 69, A-4040 Linz, Austria. Received May 25, 2005. In Final Form: June ...
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Directed Assembly of Au Nanoparticles onto Planar Surfaces via Multiple Hydrogen Bonds Ronald Zirbs,† Ferry Kienberger,‡ Peter Hinterdorfer,‡ and Wolfgang H. Binder*,† Institute of Applied Synthetic Chemistry, Division of Macromolecular Chemistry, Vienna University of Technology, Getreidemarkt 9/163/MC, A-1060 Wien, and Institute for Biophysics, Johannes Kepler University of Linz, Altenbergerstrasse 69, A-4040 Linz, Austria Received May 25, 2005. In Final Form: June 29, 2005 We have developed a new concept to effect nanoparticle binding on surfaces by use of directed, specific molecular interactions. Hamilton-type receptors displaying a binding strength of ∼105 M-1 were covalently fixed onto self-assembled monolayers via Sharpless-type “click” reactions, thus representing an efficient method to control the densities of ligands over a range from low to complete surface coverage. Au nanoparticles covered with the matching barbituric acid receptors bound with high selectivity onto this surface by a self-assembly process mediated by multiple hydrogen bonds. The binding process was investigated with atomic force microscopy. Moderate control of particle density was achieved by controlling the receptor density on the self-assembled monolayer surface. The method opens a general approach to nanoparticle and small object binding onto patterned surfaces.

Introduction Self-assembled monolayers (SAMs) offer an excellent model system to study the binding of molecular-sized objects onto surfaces.1 In particular, the binding of small molecules has been studied extensively2 using specific supramolecular interactions, featuring even molecular resolution of the binding effects with atomic force microscopy (AFM).3 Hydrogen-bonding systems,4 metal chelates,5 and hydrophobic interactions (i.e., cyclodextrinadamantane interactions)6 of sufficient strength allow the positioned binding of molecules with a high struc* To whom correspondence should be addressed. Phone: ++43-1-58801-16274. Fax: ++43-1-58801-16299. E-mail: wbinder@ mail.zserv.tuwien.ac.at. † Vienna University of Technology. ‡ Johannes Kepler University of Linz. (1) (a) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (b) Flink, S.; van Veggel, F. C: J. M.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315-1328. (c) Love, J. C.; Estroff, L. A.; Kribel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103-1169. (2) Li, X.-M.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2004, 14, 2954-2971. (3) (a) Eckel, R.; Ros, R.; Decker, B.; Mattay, J.; Anselmetti, D. Angew. Chem. 2005, 117, 489-492. (b) Zhou, S.; Scho¨nherr, H.; Vancso, G. J. Angew. Chem. 2005, 117, 978-981. (c) Scho¨nherr, H.; Beilen, M. W. J.; Bu¨gler, J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 4963-4967. (4) (a) Garcia-Lopez, J. J.; Zapotoczny, S.; Timmermann, P.; van Veggel, F. C: J. M.; Vancso, J.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Commun. 2003, 352-353. (b) Manen, H.-J.; Paraschiv, V.; GarciaLopez, J. J.; Scho¨nherr, H.; Zapotoczny, S.; Vancso, G. J.; Crego-Calama, M.; Reinhoudt, D. N. Nano Lett. 2004, 3, 4, 441-446. (c) Corbellini, F.; Mulder, A.; Ludden, M. J. W.; Casnati, A.; Ungaro, R.; Huskens, J.; Crego-Calama, M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2004, 126, 175017058. (d) Zhou, S.; Zhang, Z.; Fo¨rch, R.; Knoll, W.; Scho¨nherr, H.; Vancso, G. J. Langmuir 2003, 19, 8618-8621. (e) Credo, G. M.; Boal, A.; Das, K.; Galow, T. H.; Rotello, V. M.; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2002, 124, 9036-9037. (f) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1998, 120, 7328-7336. (5) van Manen, H.-J.; Auletta, T.; Dordi, B.; Scho¨nherr, H.; Vancso, G. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Funct. Mater. 2002, 12, 811-818. (6) (a) Mulder, A.; Auletta, T.; Sartori, A.; Del Chiotto, S.; Casnati, A.; Ungaro, R.; Huskens, J.; Reinhoudt, D. A. J. Am. Chem. Soc. 2004, 126, 6627-6636. (b) Auletta, T.; Dordi, B.; Mulder, A.; Sartori, A.; Onclin, S.; Bruinink, C. M.; Peter, M.; Nijhuis, A.; Beijleveld, H.; Scho¨nherr, H.; Vancso, G. J.; Casnati, A.; Ungaro, R.; Ravoo, B. J.; Huskens, J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2004, 43, 369-373.

tural variability onto SAMs, given that the strength of the interaction is sufficiently high. Thus, two different concepts can be followed to effect efficient binding: (a) the use of interactions where the association constant is high (usually above 105 M-1)4d and (b) the use of multiple weak interactions within one molecule, thus taking advantage of the multiplicity effects of multiple binding interactions.7 The concepts of binding molecules and larger objects onto planar surfaces has been extended to dendritic systems8a-d and polymers.8e-g The attachment of nanoparticles (NPs) onto planar surfaces is an important prerequisite to exert their function on matrixes for permanent application in the field of nano(bio)technology.9 Due to the wide range of size-dependent physical effects which can be exerted by NPs (i.e., optical,10 electrical,11 and magnetic12), a simple and general attachment strategy of the NPs on surfaces via self-assembly processes is desirable. Important for the binding process is a sufficient stabilization of the NP on the surface, combined with a high degree of binding selectivity. Besides nondirectional binding modes (i.e., (7) (a) Mammen, M.; Choi, S. K.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 2755-2794. (b) Mulder, A.; Huskens, J.; Reinhoudt, D. N. Org. Biomol. Chem. 2004, 2 (23), 3409-3424. (8) (a) Nijhuis, C. A.; Huskens, J.; Reinhoudt, D. N. J. Am. Chem. Soc. 2004, 126, 12266-12267. (b) Huskens, J.; Deij, M. A.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2002, 41, 4467-4471. (c) van Manen, H.-J.; Auletta, T.; Dordi, B.; Scho¨nherr, H.; Vancso, G. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Funct. Mater. 2002, 12, 811-817. (d) Degenhart, G. H.; Dordi, B.; Scho¨nherr, H.; Vancso, G. J. Langmuir 2004, 20, 6216-6224. (e) Norsten, T. B.; Jeoung, E.; Thibault, R.; Rotello, V. M. Langmuir 2003, 19 (17), 7089-093. (f) Sanyal, A.; Norsten, T. B.; Uzun, O.; Rotello, V. M. Langmuir 2004, 20 (14), 5958-5964. (g) Zhou, S.; Scho¨nherr, H.; Vancso, G. J. Angew. Chem. 2005, 117, 978-981. (9) (a) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 60486108. (b) Nanobiotechnology; Niemeyer, C. M., Mirkin, C. A., Eds.; WileyVCH: New York, 2004. (c) Katz, E.; Shipway, A. N.; Willner, I. In Nanoscale Materials; Liz-Marzan, L. M., Kamat, V. P., Eds.; Kluwer Academic Publishers: Norwell, MA, 2003; pp 317-343. (d) Niemeyer, C. Angew. Chem., Int. Ed. 2001, 40, 4128-4158. (e) Binder, W. H. Angew. Chem., Int. Ed. 2005, ASAP. (10) (a) Hazarika, P.; Ceyhan, B.; Niemeyer, C. M. Angew. Chem., Int. Ed. 2004, 43, 6469-6471. (b) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849-1862. (11) Granot, E.; Fernando, P.; Willner, I. J. Phys. Chem. B 2004, 108, 5875-5881. (12) Yellen, B. B.; Friedman, E. Adv. Mater. 2004, 16, 111-115.

10.1021/la051387s CCC: $30.25 © 2005 American Chemical Society Published on Web 08/02/2005

Assembly of Au Nanoparticles onto Planar Surfaces

interfacial ordering effects,13 controlled solvent evaporation,14 and covalent binding strategies15), most known methods rely on intermolecular ordering forces to mediate binding of NPs onto surfaces. Most important to this purpose are purely electrostatic interactions, which allow an efficient binding of NPs onto planar surfaces16 and other nanosized objects (i.e., DNA,17 S-layer proteins,18 carbon nanotubes,19 and viruses20). Layer by layer deposition methods21 between charged polymers and NPs can also lead to selective deposition into specific surface layers. Steric stabilization forces can lead to gradients of NPs on patterned surfaces.22 Supramolecular ordering principles of NPs on surfaces rely on directed intermolecular forces. The complementarity of oligonucleotides can be used to organize nanoparticles onto single-stranded DNA presented from self-assembled monolayers.23 Biogenic receptors such as DNA24 provide a highly selective binding scaffold. Pd pincer complexes have been used to position Au nanoparticles via Pd-phosphine interactions.7b Hydrogen bonds relying on thymine-triazine interactions have been used to attach Au nanoparticles into ordered arrays.25 To obtain stable binding of the NP to a surface, the forces between the NPs and the surface need to be controlled. Thus, the association constant between supra(13) (a) Duan, H.; Wang, D.; Kurth, D. G.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2004, 43, 5639-5642. (b) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226-229. (c) Reincke, F.; Hickey, S. G.; Kegel, W. H.; Vanmaekelbergh, D. Angew. Chem., Int. Ed. 2004, 43, 458-462. (14) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (15) (a) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (b) Garno, Y. Yang, J. C.; Amro, N. A.; Cruchon-Dupeyrat, S. Chen,S.; Liu, G.-Y. Nano Lett. 2003, 3, 389-395. (c) Hoeppener, S.; Maoz, R.; Cohen, S. R.; Chi, L.; Fuchs, H.; Sagiv, J. Adv. Mater. 2002, 14, 1036-1041. (d) Chen, S. Adv. Mater. 2000, 12, 186. (e) Yamanio, Y.; Yonezawa, T.; Shirahata, N.; Nishihara, H. Langmuir 2004, 20, 1054-1056. (f) Schmid, G.; Beyer, N. Eur. J. Inorg. Chem. 2000, 835837. (16) For two excellent recent reviews see: (a) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293-346. (b) Maenosono, S.; Okubo, T.; Yamaguchi, Y. J. Nanopart. Res. 2003, 5, 5-15. (c) Mizuno, M.; Sasaki, Y.; Yu, A. C. C.; Inoue, M. Langmuir 2004, 20, 11305-11307. (d) Gole, A.; Orendorff, C. J. Murphy, C. J. Langmuir 2004, 20, 7117-7122. (e) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T Gedanken, A. J. Phys. Chem. B 2004, 108, 4046-4052. (f) Li, X.-M.; Paraschiv, V.; Huskens, J.; Reinhoudt, D. N. J. Am. Chem. Soc. 2003, 125, 42794284. (g) Lee, K.; Pang, F.; Carroll, G. T.; Turro, N. J.; Koberstein, J. T. Langmuir 2004, 20, 1812-1818. (h) Tanaka, H.; Mituishi, M.; Miyashita, T. Langmuir 2003, 19, 3103-3105. (i) Maenosono, S.; Okubo, T.; Yamaguchi, Y. J. Nanopart. Res. 2003, 5, 5-15. (j) Bhat, R. R.; Fischer, D. A.; Genzer, J. Langmuir 2002, 18, 5640-5643. (k) Kolny, J.; Kornowski, A.; Weller, H. Nano Lett. 2002, 2, 361-364. (l) SheeneyHai-Ichida, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78-83. (m) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789. (n) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2000, 39, 1180. (o) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258. (l) Carroll, J. B.; Frankamp, B. L.; Srivastava, S.; Rotello, V. M. J. Mater. Chem. 2004, 14 (4), 690-694. For an example of nanoparticle binding onto polymeric surfaces see: Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 1595015951. (17) Woehrle, G. H.; Warner, M. G.; Hutchison, J. E. Langmuir 2004, 20, 5982-5988. (18) (a) Gyo¨rvary, E. S.; O’Riordan, A.; Quinn, A. J.; Redmond, G.; Pum, D.; Sleytr, U. W. Nano Lett. 2003, 3, 315-319. (b) Bergkvist, M.; Mark, S. S.; Yang, X.; Angert, E. R.; Batt, C. A. J. Phys. Chem. B 2004, 108, 8241-8248. (c) Hall, S. R.; Shenton, W.; Engelhardt, H.; Mann, S. ChemPhysChem 2001, 3, 184-186. (19) (a) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M. Siegel, R. W. Nano Lett. 2003, 3, 275-277. (b) Katz, E.; Willner, I. ChemPhysChem 2004, 5, 1084-1104. (20) Knez, M.; Sumser, M. P.; Bittner, A. M.; Wege, C.; Jeske, H.; Hoffmann, D. M.; Kuhnke, K.; Kern, K. Langmuir 2004, 20, 441-447. (21) Carrara, M.; Kakkassery, J. J.; Abid, J.-P.; Fermin, P. J. ChemPhysChem 2004, 5, 571-575. (22) Bhat, R. R.; Genzer, P.; Chaney, B. N.; Sugg, H. W.; LiebmannVinson, A. Nanotechnology 2003, 14, 1145-1152. (23) Andersson, M.; Elihn, K.; Fromel, K.; Caldwell, K. D. Colloids Surf., B 2004, 34, 165-171.

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Figure 1. Concept of nanoparticle binding mediated by Hamilton-type (cleft) receptors.

molecular receptors located between the nanoparticles and the surface should be sufficiently strong and is often increased by multivalency effects. In this respect Reinhoudt et al.8b,26 have used cyclodextrins as molecular printboards for the attachment of silica particles. Whereas the isolated receptor-ligand interaction with an asociation constant of Kassn ) 5 × 104 M-1 was not sufficient to mediate a selective binding process of the NPs onto the surface, the prelayering with dendrimers increased the multivalency of the binding and thus allowed a dense packing of silica particles onto the particle surface. Our approach toward NP binding onto surfaces relies on directed-hydrogen-bonding systems (Figure 1).27 The choice of the multivalent, Hamilton-type receptor28 was made according to previous experiments in bulk materials,29 where this “cleft”-type receptor showed sufficient stability for the buildup of supramolecular layered structures. The association constants of the Hamilton-type (24) (a) Demer, L. M.; Ginger, D. S.; Park, S.-J.; LI, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836-1838. (b) Cao, Y.; Jin, R.; Mrikin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (c) Park, S.; Lazarides, A. A.; Mirkin, C: A.; Brazis, P. W.; Kannewurf, C. R.; Letsinger, R. L. Angew. Chem., Int. Ed. 2001, 40, 2909. (d) Park, S.; Lazarides, A. A.; Mirkin, C. A.; Brazis, P. W.; Kannewurf, C. R.; Letsinger, R. L. Angew. Chem., Int. Ed. 2000, 39, 3845. (e) Loweth, C. J.; Caldwell, W. B.; Peng, X.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808. (f) Taton, T. A.; Mucic, R. C.; Mirkin, C: A.; Letsinger, R. L. J. Am. Chem. Soc. 2000, 122, 6305. (g) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849 and references therein. (h) Demers, L. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 3069. (25) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Rotello, V. M. Nature 2000, 404, 476. (b) Boal, A. K:; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4917. (c) Boal, A. K.; Rotello, V. M. Langmuir 2000, 16, 9527. (d) Kunz, M. J. Ph.D. Thesis, Vienna University of Technology, Vienna, 2004. (26) (a) Mahalingam, V.; Onclin, S.; Peter, M.; Ravoo, B. J.; Huskens, J.; Reinhoudt, D. N. Langmuir 2004, 20, 11756-11762. (b) Huskens, J.; Deij, M. A.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2002, 41, 44674471. (c) Onclin, S.; Huskens, J.; Ravoo, B. J.; Reinhoudt, D. N. Langmuir 2004, 20, 13, 5460-5466. (d) Mulder, A.; Onclin, S.; Peter, M.; Hoogenboom, J. P.; Beijleveld, H.; ter Maar, J.; Garcia-Parajo, M. F.; Ravoo, B.; Huskens, J.; van Hulst, N. F.; Reinhoudt, D. N. Small 2005, 1 (2), 242-253. (27) (a) Farnik, D.; Kluger, C.; Kunz, M. J.; Machl, D.; Petraru, L.; Binder, W. H. Macromol. Symp. 2004, 217, 247-266. (b) Binder, W. H. Monatsh. Chem. 2005, 1-17. (c) Binder, W. H.; Bernstorff, S.; Kluger, C.; Petraru, L.; Kunz, M. J.; Torma, V. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2004, 45, 620-621. (d) Berl, V.; Schmutz, M.; Krische, M. J.; Khoury, R.; Lehn, J.-M. Chem.sEur. J. 2002, 8, 1227-1244. (28) (a) Tecilla, P.;. Hamilton, A. D. J. Chem. Soc., Chem. Commun. 1990, 1232-1234. (b) Chang, S.-K.; Hamilton, A. D. J. Am. Chem. Soc. 1988, 110, 1318-1319. (c) Chang, S.-K.; van Engen, D.; Hamilton A. D. J. Am. Chem. Soc. 1991, 113, 7640-7645. (e) Other association constants of Hamilton receptors are described in ref 27d. (29) (a) Binder, W. H.; Kunz, M. J.; Ingolic, E. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 162-172. (b) Binder, W. H.; Kunz, M. J.; Kluger, C.; Hayn, G.; Saf, R. Macromolecules 2004, 37, 1749-1759. (c) Binder, W. H.; Kluger, C. Macromolecules 2004, 37, 9321-9330. (d) Kunz, M. J. Ph.D. Thesis, Vienna, 2004.

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receptors vary between 2.1 × 104 and 1.4 × 106 M-1,28b thus presenting multiple hydrogen bonds on a planar surface that can interact selectively with the matching receptor on the particle, mediating a highly selective binding process of Au NPs to the (Au) surface. Additionally, we report on the modular generation of surfaces with hydrogen-bonding receptors and the stable binding of the NPs onto the derivatized surfaces as detected by AFM. The coverage of NPs can be roughly adjusted by the receptor concentration used to modify the surface, scaling thereby from sparse coverage to densely packed NPs. In the present study we have tested the binding onto the surface using two sizes of NPs, namely, those with a diameter of 5 nm (where the binding is mediated by only a few (∼3) receptors) and those with a diameter of 20 nm (where the binding event is mediated by many (∼45) receptors). Experimental Section Synthesis. 11-Azidoundecanethiol (1) was prepared according to previously published syntheses.30a Product 1 was not stable over longer periods of time and thus had to be used directly for the Au NP preparations. The purity and chemical integrity of all samples was proven by 1H and 13C NMR as well as mass spectrometry and elemental analysis. The solvents used for the preparation of the SAMs were of highest purity (i.e., 99.9%); solvents used for organic synthesis were purified by distillation before use and were bubbled with argon before use, thus achieving a purity of more than >99% as checked by gas chromatography. The barbituric acid derivative 9 for assembly onto gold NPs was prepared by known procedures.4f 5-Ethyl-1,3-dimethyl-5-(10mercaptoundecyl)pyrimidine-2,4,6-(1H,3H,5H)-trione (8) was prepared in a three-step procedure following procedures reported in the literature.4f,30d 5-Ethyl-1,3-dimethyl-5-undecenylpyrimidine-2,4,6(1H,3H,5H)-trione (7). A solution of 5-ethyl-5-undecenylpyrimidine-2,4,6-(1H,3H,5H)-trione4f (6) (0.1 g, 0.36 mmol), NaOH (30 mg, 0.72 mmol), and diisopropyl ether (1 mL) was stirred in water (1.5 mL). After addition of dimethyl sulfate (150 mg, 1.15 mmol) the reaction was stirred at room temperature for 4 h.30d The solvent was removed by evaporation to yield the crude product (115 mg, 94%), which was purified by silica gel chromatography (eluent 1/10 ethyl acetate/hexane) to yield 105 mg (87%, 0.31 mmol) of final, pure 5-ethyl-1,3-dimethyl-5undecenylpyrimidine-2,4,6-(1H,3H,5H)-trione (7): 1H NMR (CDCl3, 400 MHz) δ 0.72 (t, 3H, J ) 7.4 Hz), 1.17 (m, 16H), 1.97 (m, 6H), 3.3 (s, 6H), 4.91 (m, 2H), 5.75 (m, 1H); 13C NMR (CDCl3, 50 MHz) δ 9.43, 25.20, 28.33, 28.37, 28.80, 28.97, 29.08, 29.26, 29.30, 29.37, 33.37, 33.71, 39.72, 57.52, 114.05, 139.10, 151.20, 172.04. Anal. Calcd for C19H32O3N2: 67.82 (C), 9.59 (H), 8.33 (N). Found: 67.75 (C), 9.50 (H), 8.20 (N). 5-Ethyl-1,3-dimethyl-5-[10′-(S-thioacetyl)undecyl]pyrimidine-2,4,6-(1H,3H, 5H)-trione. A 105 mg sample of 7 (0.31 mmol) and 30 µL of freshly distilled thioacetic acid (31.8 mg, 0.42 mmol) were dissolved in CDCl3 (1 mL) in an NMR tube and irradiated for 2 h with a 450 W medium-pressure mercury lamp.4f Concentration in vacuo gave 115 mg (90%, 0.28 mmol) of pure 5-ethyl-1,3-dimethyl-5-[10′-(S-thioacetyl)undecyl]pyrimidine2,4,6-(1H,3H,5H)-trione: 1H NMR (CDCl3, 200 MHz) δ 0.70 (t, 3H, J ) 7.5 Hz), 1.15 (m, 16H), 1.51 (m, 2H), 1.95 (m, 4H), 2.27 (s, 3H), 2.81 (t, 2H, J ) 7.5), 3.29 (s, 6H); 13C NMR (CDCl3, 50 MHz) δ 9.42, 25.17, 28.29, 28.33, 28.66, 29.28, 33.32, 39.68, 39.79, 57.46, 151.14, 171.95, 195.88. 5-Ethyl-1,3-dimethyl-5-(10′-mercaptoundecyl)pyrimidine2,4,6-(1H,3H,5H)-trione (8). According to Myles et al.,4f acetyl chloride (1 mL) was added slowly to methanol (10 mL) at 0 °C. After addition of the 5-ethyl-1,3-dimethyl-5-[10′-(thioacetyl)undecyl]pyrimidine-2,4,6-(1H,3H,5H)-trione (115 mg, 0.28 mmol) (30) (a) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051-1053. (b) Thaxton, C. S.; Mirkin, C. A. In Nanobiotechnology; Niemeyer, C. M., Mirkin, C. A., Eds.; Wiley-VCH: New York, 2004; pp 288-307. (c) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (d) Jursic, B. S. Tetrahedron Lett. 2000, 5325-5328.

Zirbs et al. the solution was stirred for 3 h at 0 °C. After being warmed to room temperature the solution was stirred for another 16 h. After addition of 50 mL of water, the solution was extracted with three (25 mL) portions of ethyl acetate. The combined organic phases were collected, dried over sodium sulfate, filtered, and concentrated in vacuo. The residue was quickly purified by chromatography over silica gel (hexane/ethyl acetate ) 3/1) to yield 95 mg (84%, 0.26 mmol) of pure product 8. The product was not stable over longer periods of time and thus was used directly for the Au NP preparations. Data for 8: 1H NMR (CDCl3, 400 MHz) δ 0.78 (t, 3H, J ) 7.5 Hz), 1.05 (m, 1H), 1.22 (m, 16H), 1.68 (m, 2H), 2.03 (m, 4H), 2.69 (t, 2H, J ) 7.5), 3.36 (s, 6H); 13C NMR (CDCl3, 50 MHz) δ 9.52, 25.30, 28.50, 29.19, 29.42, 33.44, 39.11, 39.79, 57.59, 151.14, 172.08. Anal. Calcd for C19H34O3N2S1: 61.59 (C), 9.25 (H), 7.56 (N). Found: 61.50 (C), 9.21 (H). 5-(5-Hex-5-ynoylamino)-N,N′-bis(6-octanoylaminopyridin-2-yl)isophthalamide (4). The Hamilton-type cleft receptor 4 was prepared according to the standard procedure developed in our laboratory and published previously.29c A solution of 5-amino-N,N′-bis(6-octanoylaminopyridin-2-yl)isophthalamide (5)29c (0.5 g, 0.8 mmol) in dry THF (15 mL) was cooled to 0 °C. After addition of N,N′-diisopropylethylamine (0.16 mL, 0.88 mmol) a solution of hexynoic acid chloride (0.117 g, 0,88 mmol) in dry THF (5 mL) was slowly added to the reaction mixture. The cooling bath was removed after 2 h, and the solution was stirred overnight. After a standard workup procedure29c 0.55 g of pure 4 was obtained: 1H NMR (400 MHz, DMSO-d6) δ 10.43 (s, 3H), 10.16 (s, 2H), 8.41 (s, 2H), 8.26 (s, 1H), 7.87 (m, 6H), 2.90 (s,1H), 2.44 (m, 6H), 2.31 (dt, 2H), 1.84 (m, 2H), 1.62 (m, 4H), 1.31 (m, 16H), 0.89 (t, 6H); 13C NMR (50 MHz, DMSO-d6) δ 173.19, 172.12, 166.13, 151.56, 151.04, 141.07, 140.75, 135.71, 122.74, 122.40, 111.18, 110.93, 84.92, 72.71, 37.09, 36.05, 32.15, 29.47, 25.98, 24.81, 23.07, 18.31, 14.92. Au NPs. Dispersions of protected gold colloids were prepared using published procedures30b,c and subsequent incubation with the corresponding thiols.4f Biefly, the 20 nm Au NPs were prepared via the citrate reduction method30b in water followed by centrifugation, washing with diluted sodium hydroxide solution (0.1 mM) to remove nonspecifically adsorbed ligands, and immediate immersion in the thiol solution (3, 8, or 9) in ethanol (1 mM), centrifuged, and resuspended in toluene (final solution ∼5 mg of Au/10 mL). The 5 nm Au NPs30c were prepared by phase-transfer catalysis (tetraoctylammonium iodide) via sodium borohydride reduction in a biphasic (toluene/water) mixture in the presence of the thiol ligand 3, 8, or 9. After collection of the toluene phase, the organic phase was diluted with dry ethanol (200 mL) and stored at -25 °C for 20 h and the resulting NP precipitate was collected by decantation. Resuspension in ethanol was repeated two times, and finally, the NPs were suspended in toluene (final concentration ∼20 mg of Au/10 mL). All NPs were stored in polypropylene tubes in the dark to prevent flocculation and oxidation. The size was determined by dynamic light scattering and controlled by transmission electron microscopy (TEM) measurements (see the Supporting Information). Preparation of SAMs. All used substrates (Au(111) on mica) were made from cleaved mica with epitaxially grown gold at least 1500 Å thick. The wafers were flame-annealed and packed under nitrogen prior to shipping. Immediately prior to use they were washed in ethanol (3 × 20 mL), blow-dried in high-purity nitrogen (1 min), and exposed to a UV/ozone atmosphere in a commercial cleaning chamber (Boeckel Industries, model UV clean) for 20 min. The mixed SAMs were produced by immersion of the cleaned gold surfaces in different deposition solutions for 24 h at 45 °C to achieve equilibration, leading to homogeneously mixed monolayers.31 These solutions were prepared by dissolving the desired amount of thiols in ethanol. The total concentration of thiols was always 1 mM. After the deposition the SAMs were washed in ethanol (3 × 20 mL) and water (2 × 20 mL), blow dried with high-purity-nitrogen, and stored under nitrogen until needed for further measurements. The thickness of the SAMs was (31) (a) Weisbecker, C.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772. (b) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. Langmuir 2000, 16, 9287-9293. (c) Stevade, A. V.; Zhou, J.; Zin, M. T.; Jiang, S. Langmuir 2001, 17, 7566-7572.

Assembly of Au Nanoparticles onto Planar Surfaces obtained by ellipsometric measurements (PLASMOS SD 2300 ellipsometer). Optical constants of n ) 0.1850 and k ) -3.5263 for gold and n ) 1.5 and k ) 0 for the organic films were used. To bind the receptor onto the surface by 1,3-dipolar cycloaddition reactions (Sharpless “click” reactions), the mixed SAMs were completely submerged in a solution of the receptor alkyne 4 (150 mg, 0.2 mmol) and copper(II) sulfate pentahydrate (2.4 mg, 9.6 µmol) in DMF (4 mL). A solution of sodium ascorbate (24 mg, 0.12 mmol) in water (2 mL) was added, and the turbid solution was homogenized and stirred at 25 °C for 48 h (all solutions were degassed with argon to prevent dimerization of the alkyne or oxidation of the Cu(I); the sodium ascorbate acts as a reducing agent, thus generating the catalytically active Cu(I) species in situ). Different mixtures of solvents and catalysts were tested for this reaction (ethanol/water, methanol/water, toluene; (PPh3)3CuII, CuIBr, CuII, CuIISO4 plus sodium ascorbate): the described method using CuIISO4 plus sodium ascorbate showed the best results in the shortest reaction time at room temperature. The reaction between CuIISO4 and sodium ascorbate generates a Cu(I) species in situ as described by Sharpless et al.34c Reactions with (PPh3)3CuI in toluene were not successful, presumably due to reduction of the azide moieties by triphenylphosphine (Staudinger reduction). Final washing of the SAMs (4 × 20 mL of water, followed by 5 × 20 mL of ethanol, each for 2 min) and blow drying with nitrogen furnished the final, modified SAMs. Ellipsometry/IR Spectroscopy/Dynamic Light Scattering (DLS). The reaction progress was evaluated by ellipsometric measurements and grazing angle infrared spectroscopy. Infrared spectra of the produced monolayers were obtained using a surfaceenhanced reflection technique, were the monolayer-covered wafer was pressed against a hemispherical germanium crystal and a p-polarized IR beam was directed through the Ge crystal onto the sample surface at 65° incidence. DLS measurements were done in toluene solutions of the NPs after dilution by ∼1/50 with pure toluene. DLS was performed on an ALV/CGS-3 compact goniometer, using the ALV-5000/E correlator software. Deposition of Nanoparticles on SAMs. The SAMs were incubated with ∼5 mL of the corresponding NP solution in toluene for the period indicated (maximum incubation time 40 h). The resulting samples were subsequently washed with pure toluene (3 × 20 mL) to remove NPs and directly measured with AFM while suspended in toluene. AFM Measurements. All topographical images shown were acquired in toluene with a magnetically driven MACmode atomic force microscope (Molecular Imaging, Phoenix, AZ) using magnetically coated MacLevers (Molecular Imaging). Prior to imaging, amplitude-distance cycles were used to adjust the free cantilever oscillation amplitude to ∼8 nm and to determine the optimal amplitude reduction value (i.e., the set point value) to ∼10%. Both the low value of the free oscillation amplitude (8 nm) and the low amplitude reduction (10%) used as feedback signals are of particular importance for preventing disruption of the sample and stable imaging.32,33 The oscillation frequency of the cantilever was adjusted to ∼7 kHz using 100 pN/nm spring constant levers. Images were taken at a line scan rate of ∼1 Hz, and 512 pixels/line were recorded. The image acquisition time was ∼9 min. The surface coverage of the receptors was determined from topographical images using a threshold algorithm; i.e., a threshold was set, and the percentage of pixels above this value was calculated. In detail, a lower set point and a higher set point were made, and the resulting covered surface was evaluated using the Photoshop program. This procedure was efficient for the determination of the area coverage of the receptor 4. The number of NPs per unit area was counted manually. Thus, the number of NPs was determined by handcounting of several smaller surface areas (500 nm × 500 nm) with subsequent statistical averaging.

Results and Discussion Preparation of SAMs. To generate a multitude of different surfaces starting from simple precursors, a postmodification strategy was developed, relying on the (32) Han, W.; Lindsay, S. M.; Jing, T. Appl. Phys. Lett. 1996, 69, 1-3. (33) Kienberger, F.; Zhu, R. Moser, R. Rankl, C.; Blaas, D.; Hinterdorfer, P. Biol. Proced. Online 2004, 6, 120-128.

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Figure 2. Preparation of functionalized SAM surfaces on gold. Molecularly flat Au surfaces were first incubated with a solution of the thiol 1, 2, or 3 (either pure or mixtures of thiol 1 and 2, ranging from 0.1 to 100 mol % 1). The prepared surfaces were then reacted with cleft receptor 4 in a DMF/water mixture and 1 mol % CuSO4/ascorbate for 10 h. Rinsing of the surface and subsequent drying furnishes the derivatized surfaces.

recently published Sharpless-type click reaction.34 We have chosen this two-step method to (a) prepare mixed SAMs by immersion in one reactive thiol (e.g., the ω-azidothiol 1) and one nonreactive thiol (hexylthiol (2)) and (b) subsequently fix the Hamilton receptor 4 onto this SAM. This enables the stepwise generation of mixed SAMs with a more simplified variation of the fraction of functional Hamilton receptor 4 on the SAM. The Sharpless click reaction has been demonstrated as an extremely efficient reaction on SAM surfaces in the past,30a,34a,b thus representing the method of choice in our case. Surface functionalization (see Figure 2) was initiated by presenting terminal azido moieties on Au wafers on the basis of SAMs. The pure Au surfaces were reacted with the ω-azidothiol 1, either in pure form or in a mixture with 2, thus presenting terminal azido moieties, which can be detected by grazing angle IR spectroscopy (see the Supporting Information). The reaction was run at elevated temperatures (∼45 °C) to ensure a homogeneously mixed SAM of these two components.31b The terminal azido moieties were then derivatized with the multiple-hydrogenbonding receptor 4 bearing terminal acetylenic moieties via a 1,3-dipolar cycloaddition processsa reaction type which reportedly runs at room temperature with high efficiency.29b The loss of the azido moiety at 2103 cm-1 and the concomitant growth of the band at 1540 cm-1 (aryl and amide bands of 4) demonstrate the progress of this click reaction, thus fixing the cleft receptor 4 onto the SAM surface. The most important parameter during this reaction was the choice of the copper(I) catalyst. Too high a concentration of the conventionally used catalyst tris(triphenylphosphine)copper(I) bromide ((PPh3)3CuIBr) led to the decomposition of the azido moieties at the SAM surface via Staudinger-type reactions.35 The optimal (34) For 1,3-dipolar cycloaddition reactions on surfaces see: (a) Lummerstorfer, T.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 39633966. (b) Lee, J. K.; Chi, Y. S.; Choi, I. S. Langmuir 2004, 20, 38443847. (c) two reviews on this reaction: Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004-2021. (d) Binder, W. H.; Kluger, C. Curr. Org. Chem., in press. (35) Ko¨hn, M.; Breinbauer, R. Angew. Chem., Int. Ed. 2004, 43, 31063116.

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Scheme 1. Synthesis of the Cleft Hamilton Receptor 4 and Thiol 8

catalyst system was based on CuIISO4/ascorbate in a mixture of N,N-dimethylformamide/water at a concentration of 1.7 × 10-3 M, leading to a reaction time of ∼20 h. Ellipsometric measurements confirmed the reaction as measured by the increase in layer thickness (starting from 1.2 ( 0.1 nm (a SAM with 100 mol % thiol 1) to 2.8 ( 0.1 nm after the click reaction with the alkyne 4, full data in the Supporting Information), with a calculated thickness of ∼2.5 nm assuming a tilting angle of approximately 28°.1a Synthesis of the Hamilton receptor bearing the terminal acetylenic moiety (4) was accomplished in a simple acylation sequence starting from the known amine 5 as described in the literature29c,36 (Scheme 1). A receptor not able to interact with the Hamilton receptor (N,N′dimethyl-ω-thioalkylbarbiturate 8) was prepared as a control molecule for later NP adsorption studies. The density of the receptors at the surface could be controlled by use of the ω-azidothiol 1 in mixed SAMs on Au with the nonreactive 2, thus achieving final area coverages of the receptor 4 between 3% and 100%. Nonreactive surfaces (i.e., those devoid of a receptor) were prepared by use of ω-octadecylthiol (3). Topographical images of the derivatized surfaces are shown in Figure 3. Receptor molecules can be seen “sticking out” of the surface by about 3.0-3.5 nm (Figure 3B-D), which nicely matches the molecular dimension of the receptor structure (∼3.0 nm; see Figure 2). Similar observations of receptors sticking out of a SAM surface have been recorded by Reinhoudt et al. as well as Vancso et al.5,37 with Pd pincer complexes. Since the receptor density is important for the subsequent NP binding process, different surfaces with an initial solution concentration ranging from 0.1 to 100 mol % were prepared and imaged using AFM. The area coverage of receptor 4 was observed to range from low densities to fully covered surfaces; i.e., a surface initially prepared from 0.1 mol % 1 showed an area coverage of receptor 4 of ∼3% (Figure 3B), 1 mol % 1 led to ∼53% area (36) Zhang, S.-Q.; Fukase, K.; Izumi, M.; Fukase, Y.; Kusumoto, S. Synlett 2001, 5, 590-596. (37) (a) Menozzi, E.; Pinalli, R.; Speets, E. A.; Ravoo, B. J.; Dalcanale, E.; Reinhoudt, D. N. Chem.sEur. J. 2004, 10, 2199-2206.

coverage of the receptor 4 (Figure 3C), and 100 mol % 1 led to 100% area coverage of the receptor 4 (Figure 3D). These values correspond well with the calculated area coverages of 4 (3%, 30%, and 100%, respectively). Almost full area coverage (98%) was already achieved with a solution containing 2 mol % ω-azidothiol 1 and subsequent click reaction with the cleft receptor 4. Thus, until saturation there is a relationship between the applied SAM mixture and the final receptor density (Figure 3E). NP Binding. Nanoparticle binding was studied using Au NPs modified30b,c with barbituric acid receptors (Figure 4) as well as those not able to yield a specific hydrogenbonding interaction (Figure 5C). Au NPs with a size of 20 nm were prepared by the citrate reduction method30b and those with a size of 5 nm by the sodium borohydride reduction method,30c followed by immersion with the corresponding ω-thiols. The nanoparticles were prepared bearing (see Figure 5A) (a) barbituric acid receptors by adsorption of the thiol 9 (NP2) (b) N,N-dimethylbarbituric acid moieties by adsorption of thiol 8 (NP3), and (c) octadecylthiol-capped NPs (NP4). The association constant between a cleft-type receptor 4 and the barbituric acid is reported to be as high as 1.4 × 106 M-1,28c although this value is diminished in oligomeric systems to >30000 M-1 27d,29d and with other substitution patterns.27d,28,29d Our determination of the association constant by NMR titration experiments in solution yielded Kassn ) 1.2 × 105 M-1 (see the Supporting Information). The particles are highly uniform as viewed by TEM measurements and do not show agglomeration (Figure 4E and Supporting Information). Binding of the 20 nm sized particles (NP2, 12-20 h until completion) to a surface presenting 100 mol % receptor 4 (corresponding to 100% area coverage) showed a densely packed layer of Au NPs (Figure 4A). The Au NPs can be clearly seen in the corresponding cross-section profiles with heights of 15-20 nm. Single NPs can be discerned on a smaller scan size image, whereas ∼300 NPs can be counted per square micrometer (Figure 4B). To demonstrate the stability of the supramolecular interaction, the same surface area was imaged consecu-

Assembly of Au Nanoparticles onto Planar Surfaces

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Figure 3. (A-D) AFM images of receptors immobilized on gold surfaces. (A) Topographical image of a SAM of 100 mol % (solution) 1 on Au before the click reaction. The cross-section profile (along the line in green) shows a surface roughness of ∼0.2 nm. (B-D) Images of monolayers consisting of (B) 0.1 mol % 1/99.9 mol % 2, (C) 1 mol % 1/99 mol % 2, and (D) 100 mol % 1 after derivatization with receptor 4. The receptor cleft molecules 4 are seen as ∼0.35 nm elevations (see the crosssection profiles and inset of (C)). (E) Graphical representation of the initial area coverage with thiol 1 (before the click reaction (0 mol % cleft 4)) and the area coverage with the cleft receptor 4 after the Sharpless click reaction (determined via a threshold algorithm). Thus, 0 mol % 1 in solution yields an area coverage of 0% of receptor 4, 0.1 mol % 1 (in solution) yields an area coverage of ∼3% of receptor 4, a surface with 1 mol % 1 (solution) yields an area coverage of ∼53% of receptor 4, a surface with 2 mol % 1 (solution) yields an area coverage of ∼>98% of receptor 4, and a surface with 100 mol % 1 (solution) yields an area coverage of 100% of receptor 4.

Figure 4. AFM images of nanoparticles NP2 at different receptor surface densities. (A) Topographical image of Au nanoparticles bound onto a surface derivatized with 100 mol % 4. The cross-section profile shows a densely packed Au NP layer along the L-shaped defect structure with a height of 1520 nm. (B) Enlarged structure: small scan-size image showing densely packed NPs (arrows) on a surface derivatized with 100 mol % 4 (the area coverage of receptor 4 is 100%). (C) Sparse coverage of NPs (arrows) obtained on a surface derivatized with 1 mol % 4 (the area coverage of receptor 4 is ∼53%). (D) Surface derivatized using 0.1 mol % receptor 4 (the area coverage of receptor 4 is ∼3%). The NPs (arrow) are only observed occasionally. Single NPs are 15-20 nm in height (see the crosssection profiles in (A) and (B)). (E) TEM picture of the 20 nm Au nanoparticles functionalized with barbituric acid receptors (NP2). (F) 3D representation of NPs on the surface shown in (A) (15-20 nm in height, blue-violet) and the receptor molecules (∼3 nm in height, green).

tively over many hours. The NPs thereby did not change their lateral position, demonstrating that they are stably bound to the surface (data not shown). Surfaces presenting a lower amount of receptors showed a reduced loading of the NPs: A sparse coverage was obtained on a surface derivatized with 1 mol % 4 (solution concentration during preparation, corresponds to ∼53% area coverage of receptor 4, Figure 4C, ∼4 NPs/µm2), and NPs were only occasionally observed on a surface modified with 0.1 mol % 4 (3% area coverage of 4, Figure 4D, ∼1 NP/µm2). The NPs bound to the surface scaled therefore with the initial concentration of receptor molecules; i.e., with increasing densities of the receptor 4 on the surface, more NPs were bound. However, still free receptor molecules are visible

on the surface besides receptors occupied by NPs (Figure 4F), showing that a certain number of free receptors not bound to NPs remain. To prove the specificity of the binding, nanoparticles bearing barbituric acid (NP2) were incubated to pure Au and octadecyl-modified Au surfaces; no NPs were observed on topographical images (data not shown). To estimate the binding energies of the NPs onto the surface, a simple model was calculated: Thus, an NP with D ≈ 20 nm is calculated with a surface of approximately 1256 nm2. One cleft-type receptor (4) (roughly 3 nm in diameter) occupies approximately 7 nm2. Therefore, about 180 receptors can be located at the NP surface. Given that about 1/4 of the NP surface can interact with the SAM

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Figure 5. (A) Preparation of Au colloids NP1 and coverage with different thiols (9, 8, 3), yielding the nanoparticles NP2-NP4. (B-D) Demonstration of the binding selectivity of different nanoparticles (D ) 20 nm) to a surface covered with 100% 4. (B) NP2: binding of Au nanoparticles covered with the matching barbituric acid receptor to a surface covered with 100 mol % 4. A dense layer of NP2 can be observed (∼300 NPs/µm2). (C) Images of 20 nm sized NPs with the nonmatching receptor (NP3) and a surface covered with 100 mol % 4. (D) Images of 20 nm sized NPs with an octadecyl surface (NP4) and a surface covered with 100 mol % 4.

Figure 6. Binding of 5 nm NPs to a surface covered with 100% 4. (A) Matching interaction between NP2 and a surface covered with 100% 4. A dense coverage of 5 nm sized NP2 is obtained. (B) Nonmatching interaction of NP3 and a surface covered with 100% 4. Almost no NPs are bound.

(due to the NP’s curvature), about 45 receptors could participate in the NP binding event of the D ) 20 nm particles. Therefore, in the case of the larger NPs definitely a multivalent interaction with the SAM surface is present. In the case of the D ) 5 nm NPs (surface area 78 nm2, 11 receptors total on the NP surface) about 3-4 receptors could participate in the binding event. In both cases the

association energy (calculated from ∆G ) -kBT ln(Kassn), ∼700 kBT (D ) 20 nm NPs) and 10 kBT (D ) 5 nm NPs)) exceeds the thermal energy, thus enabling a stable NP binding event on the SAM surface. Binding of the particles is highly selective as demonstrated in Figure 5 by incubating differently functionalized NPs (D ) 20 nm) with a surface treated with 100 mol %

Assembly of Au Nanoparticles onto Planar Surfaces

Hamilton receptor 4: Thus, hardly any unfunctionalized particles (NP1) bound to the surface (data not shown). The same holds true for Au NPs derivatized with octadecyl moieties (NP4), where no matching hydrogen-bonding interaction is possible, leading to a statistical deposition of only 10 particles to a square micrometer area (Figure 5D). In contrast, the binding of ∼300 NPs was observed on the same area with the matching receptor interaction (NP2) (Figure 5B). An important point concerned the selectivity of the binding process with a polar receptor devoid of multiple hydrogen-bonding interactions. In this respect, nanoparticles bearing the N,N′-dimethylbarbituric acid 8 were prepared (NP3). For electronic and steric reasons the association constant of this N,N′dimethylbarbiturate receptor is only ∼15 M-1 (for the determination of the association constant between the stable precursor 7 and the Hamilton receptor 4 see the Supporting Information). Thus, nanoparticles NP3 (bearing the nonmatching N,N′-dimethylbarbiturate receptor 8) were incubated with a surface presenting 100 mol % Hamilton receptor 4 (Figure 5C). Only the receptor molecules and very few NPs (30 NPs/µm2) were visible in this nonmatching case. Thus, the process is highly selective (a selectivity of a factor of 10 is observed, 300 NPs/ µm2 in Figure 5B versus 30 NPs/µm2 in Figure 5C) and can be used to selectively bind nanoparticles to surfaces. Similar experiments were performed with the 5 nm NPs using surfaces presenting 100% of the receptor 4 (see Figure 6). Thus, a dense layer of NPs bearing the matching barbiturate receptor 4 (NP2) was observed (Figure 6A), whereas only a few NPs were observed in the nonmatching interaction (NP3 with the surface, Figure 6B). Thus, selective binding is also applicable for smaller NPs. Conclusion We have studied the binding of Au NPs to surfaces functionalized with strong hydrogen-bonding interactions.

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A consequent and easy chemistry was developed relying on a 1,3-dipolar cycloaddition process, which allows the efficient functionalization of surfaces with these hydrogen-bonding receptors. Au NPs can be bound onto functionalized surfaces (100% area coverage) by direct interaction without the requirement of an additional layer mediating additional multivalent interactions. The density of NPs can be adjusted by the receptor concentration used for derivatizing the surface, as proven by topographical AFM imaging. In addition, repetitive imaging of the same area has shown the stability of the NP-modified surfaces. Thus, the developed chemical concept will allow a variety of NPs to be self-assembled using this assembly principle, also opening the possibility to assemble nanoobjects onto appropriately structured surfaces. Acknowledgment. This project was funded by the Austrian Science Foundation (Grant FWF-14844 CHE). We kindly thank Ms. Dipl. Ing. Doris Brandhuber for the TEM measurements and Prof. Hoffman and T. Lummerstorfer for providing the ellipsometry and surface IR measurements. We also thank Mr. Dipl. Ing. Christian Kluger for excellent assistance. Supporting Information Available: Graph for determining the association constant Kassn of receptor 4 with 5,5dipropylbarbituric acid, graph for determining the association constant Kassn of the nonmatching receptor 5-ethyl-1,3-dimethyl5-undecenylpyrimidine-2,4,6-(1H,3H,5H)-trione (7) with receptor 4, TEM picture of D ) 5 nm NPs, dynamic light scattering of D ) 5 nm and D ) 20 nm NPs, ellipsometric measurements of the surfaces, and grazing angle IR spectroscopy of the SAM bearing 100 mol % ω-azidothiol 4 before and after the click reaction. This material is available free of charge via the Internet at http://pubs.acs.org. LA051387S